Control Limits define the predictable process variations. In practice it cannot be known without some associated probability.

Nevertheless, Control Limits in SPC, from the founder Shewhart's point of view, should not be confused with Confidence Interval in Probability. Notably, it is a mistake to use Student t-distribution Confidence Interval to estimate SPC Control Limits.

The basics of Statistical Process Control (SPC) is to make the Process Control Limits fit within the Specifications Limits.

## Thursday, November 15, 2007

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